This invention relates to Costas loops, and more particularly to a modification of the Costas loop in suppressed-carrier receivers employed for carrier acquisition and tracking.
Frequency uncertainties in a receiver signal or in the local oscillator (LO) of a receiver attempting to acquire phase lock to the signal require special techniques to bring the receiver frequency close enough to the received frequency to acquire lock. One method that has been used is frequency sweep, either of the transmitter generating the received signal or of the receiver LO. This adds to the acquisition time since sweep rates are limited by the receiver phase lock loop bandwidths. It requires tracking of the swept frequency if the received signal is swept. (Until lock is detected and the command is sent back to the transmitter to disable the sweep.) If the receiver LO is swept, receiver complexity is increased and the sweep function further contributes to the frequency uncertainty. Finally, detectors for suppressed carrier signals can false lock on data sidebands without the right combination of sweep rate, detector filter bandwidths and data rate. This correct combination might be precluded by other considerations.
An object of this invention is to speed acquisition of phase lock to a suppressed carrier signal without the complexity and potential problems associated with alternate methods, such as frequency sweep, and to provide for tracking with a narrow bandwidth to minimize cycle skipping and to improve range rate accuracy.